OBSERVATION POST OUELLETTE, South Korea – Razor-wire close to the border, President Barack Obama today paid his first visit to the tense zone separating North and South Korea amid new nuclear tensions. He told American troops stationed nearby they are protectors of “freedom’s frontier.”

Obama shook hands and spoke briefly in the dining hall at a U.S. military camp just outside the 2.5-mile-wide Demilitarized Zone, then walked into the heavily patrolled no-man’s land to tour a small post where South Korean forces patrol just 100 yards from the demarcation line.

The president, positioned behind bulletproof glass, peered through binoculars across the line that has bisected the Korean peninsula for 60 years. He spent about 10 minutes at the observation post, looking first toward North Korea, then back to the South.

It was an unmistakable show of force to communist North Korea and its new leader at a time of diplomatic standoff. Obama underscored the Cold War symbolism by making the tour his first order of business ahead of a gathering of world leaders pledged to keep nuclear materials safe. Nuclear-armed North Korea will not attend.

The U.S. is threatening to cancel planned food aid to the North over its announcement that it will launch a long-range rocket next month, news that overshadows the gathering of world leaders committed to nuclear security that Obama will attend in Seoul.

“I could not be prouder of what you do,” Obama told smiling American troops at Camp Bonifas at the edge of the DMZ. Obama said the same is true at every U.S. military post, but “there’s something about this spot in particular.”

Obama’s visit takes place as North Koreans mark the end of the 100-day mourning period for longtime leader Kim Jong Il, who died of a heart attack in December. Since Kim’s death, son Kim Jong Un has been paying a series of high-profile visits to military units and made his own trip to the “peace village” of Panmunjom inside the DMZ earlier this month.